Abstract

A low-noise single-chip two-stage superconducting quantum interference device(SQUID) system with a double relaxation oscillationSQUID as the second stage has been realized. The system was operated in a direct voltage readout mode, with a closed loop bandwidth up to 1 MHz. Operated at 4.2 K, the white flux noise measured in flux locked loop was corresponding to an energy sensitivity of Owing to the large flux-to-voltage transfer of up to the room-temperature preamplifier noise did not dominate the overall flux noise.